How To Use AI To Design Electronic Circuits

This is the fourth installment in the Ten Tec Omni 6 plus repair series. As mentioned in the last installment, I had repaired and aligned the radio. However, I was still seeing frequency instability that was bad enough to prevent it from working with FT8.

Frequency Instability

photo of the TCXO with loose cover removed — TCXO with loose cover removed

After eliminating every other possibility, the problem came down to a damaged Temperature Controlled Crystal Oscillator (TCXO). The master time base for the Omni. It was still pretty accurate and could be adjusted onto its primary frequency, 20 MHz. Nevertheless, it drifted plus or minus about 20 cycles over a minute or two. This was enough to account for the radio’s inability to reliably encode and decode FT8 signals.

So the next question was, what could I do about it? An extensive search of the ‘net showed that the original part was simply not available. I would have to find a salvaged logic board from an Omni 6 plus.

TCXO Candidates

Ironically, TCXOs are widely available today. As I worked to align the radio, I had discovered the need for a more accurate Frequency Counter. This led to the discovery of several very accurate time base strategies. Maybe I could substitute a newer model of 20 mHz TCXO for the failed part.

CTI Oven Controlled Crystal Oscillator

photo of a GPS Disciplined Oscillator — GPS Disciplined Oscillator

Initially I thought about the CTI OSC5A2BO2 “Oven Controlled Crystal Oscillator” (OCXO) available on eBay for around $5. These salvaged parts were widely used in the Cell Phone industry and not very expensive. It was the same part used for the “mini GPSDO” I bought as a frequency counter time base.

The CTI part could be adjusted onto frequency with a simple external circuit. However, it was only available with a 10 mHz output, and was rather large. It also had big power needs since its current drain could be as much as 600 mA at 5 volts.

Raltron Temperature Compensated Crystal Oscillator

photo of the Raltron RTX0230LC TCXO — Raltron RTX0230LC TCXO

Further search turned up a more promising part: the RALTRON RTX0230LC 20 mHz TCXO. It was available on eBay for about a buck apiece. Although temperature compensated rather than oven controlled, it had the advantage of being tiny with modest power requirements. The question was, would it drive the TTL clock circuit in the Omni 6?

We’re going to need more power . . .

It didn’t take long to get some, and hook one up. Viewing the output on a scope with a TTL load showed it would be close to the right output level. Still, I would probably need to add a gain stage to guarantee success.

Artificial Intelligence to the rescue

Now, I have designed and built transistorized stages, but it’s been years and it takes some effort. Artificial Intellegence (AI) is very much in the air these days. I wondered if I could ask an AI to design such a circuit. So I prompted ChatGPT: “Simple one transistor circuit to amplify a 20 MHz signal generator.” The resulting answer was quick and amazing.

ChatGPT produced about a 40-line text response. I’ve included a PDF if you’d like to see it. It basically described a common emitter circuit to increase the output level of the TCXO.

Simple one transistor circuit to amplify a 20 MHz signal generator Download

Back to the 60’s?

I also asked ChatGPT to produce a schematic. I guess this is what they mean when they talk about AI hallucination. The drawing, looking very much like something the cartoonist Robert Crumb might have produced, looked nothing like the described circuit.

graphic of ChatGPT response to request for a schematic — ChatGPT response to request for a schematic

graphic of ChatGPT 2nd response to request for a schematic — ChatGPT response to request for a schematic

A real schematic and breadboard

graphic of the Master Clock Schematic — Master Clock Schematic

Undeterred, I used the description to draw my own schematic. It looked perfectly reasonable, so I decided to breadboard it. Oscillograms of the TXCO and amplifier output showed enough drive for the Omni clock circuit. It could finish shaping the waveform and interfacing with the TTL chip that produces the 10 mHz master clock.

photo of the buffer amp breadboard designed by an AI — Bread board of buffer amp

photo of the breadboarded buffer under test — Bread board of buffer amp

Building the tiny board

Over the next few days, I built the circuit onto a small 2 cm by 8 cm perfboard. I added a 5-volt regulator for the TCXO, and a discrete 9-volt regulator for the transistor stage. A potentiometer between the TCXO and the amplifier provided for a little bit of gain adjustment.

photo of Testing the new timebase — Testing the new timebase

Oscillogram of clock output — Testing the new timebase

It just fits and Logic board mods

photo of the New timebase mounted — New timebase mounted

There’s a small, empty space on one side of the logic board compartment. It’s just enough space to fit the new time base board. A couple of nylon standoffs secure it to the radio.

photo of Modifications to the Logic Board — Modifications to the Logic Board

12 volts taken from the now-removed faulty TXCO is fed through a 4-pin header to the new daughter board. 20 mHz output is fed through a 3-pin header and miniture coax back to the logic board.

Problem Solved.

And how does it work? Perfectly. A tiny adjustment on the Ralton TCXO allows fine adjustment to exactly 20 mHz. The master Omni clock is now 10 mHz — dead-on.

photo of the Timebase: a solid 10 mHzTimebase: a solid 10 mHz — Time base: a solid 10 mHz

Powering the radio for the first time after making this mod, everything came up the first time. And, the radio is now stable enough to transmit and receive FT8. Success!